Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, which is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
The distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems is generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
A radar level gauge system is often mounted on a so-called nozzle at the top of the tank. The nozzle may typically be a pipe that is welded to the tank and fitted with a flange at its upper end to allow attachment of an instrument, such as a radar level gauge system, or a blind flange. In such cases, the probe is typically mechanically connected to the tank at the top of the nozzle, and passes through the nozzle before entering the tank itself. At the top of the nozzle, the probe may be electrically connected to the transceiver of the radar level gauge system through a feed-through that passes through the tank boundary.
It is important that the radar level gauge system is designed and approved to include mandatory explosion protection means, since the transmitted electromagnetic signals may be fed into a tank with flammable and explosive gases and liquids. Such a hazardous environment puts restrictions on how the electromagnetic signals for transmission are formed and fed.
For a GWR system where signal wires are entering the tank a protection system called intrinsic safety is generally to be used. The concept intrinsically safe is defined by relevant norms and means that neither normal signals nor voltages possibly occurring under failure conditions should be capable of causing an ignition, even under worst case conditions. Examples of norms for intrinsic safety are IEC610079-0 and IEC610079-11. Outside tanks but close to them less stringent protection systems may be used.